Neutron generators



March 1967 J.ID. L. H. WOOD ETAL 3,311,771

NEUTRON GENERATORS Filed June 24, 1963 United States Patent 3,311,771NEUTRON GENERATORS James David London Hedley Wood and Peter DonaldLomer, Letchworth, England, assignors to the United Kingdom AtomicEnergy Authority, London, England Filed June 24, 1963, Ser. No. 290,086

Claims priority, application Great Britain, June 28, 1962,

3 Claims. (Cl. 31361) This invention relates to neutron generators ofthe type in which an ion source generates ions which are acceleratedacross an acceleration gap by a potential difference applied across thegap, so as to strike a target material with high energy, the ion sourceand the acceleration gap being within a common sealed envelope atsubstantially the same pressure. It is necessary to have a shortwelldefined acceleration gap so as to keep the conditions well up theleft hand side of the Paschen curve relating breakdown voltage to theproduct of the gas pressure and the acceleration distance.

In such generators the gas pressure is normally within the range 1 to1000x mm. pressure and the voltage across the acceleration gap isnormally 10-200 kv.

Much information on neutron generators is available in published formand attention is directed to Review of Scientific Instruments, 31 (1960)3 and to Nucleonics, vol. 18, No. 12, December 1960, pages 69-76 whichgive descriptions of some sealed-off neutron generators.

In the calibration of neutron generators employing the DT reaction, itis customary to count the 3.5 m.e.v. alpha particle from the reaction T+D a+n+ 17.6 m.e.v. Alpha particles are, however, easily ahsorbedby eventhin solid materials and the counting of alpha particles through thesealed envelope of sealed-off neutron generators is impossible. A thinwindow for passage of alpha particles through the envelope would be sothin that it would be too fragile and could not stand normal handling.

This invention provides a neutron generator of the type specified havingan alpha particle counter inside the sealed envelope.

The invention consists in a neutron generator of the type specified inwhich an electrostatic shield is positioned between the target and theion source to provide one boundary of the acceleration gap and maintaina fieldfree zone within which the target is located, sa-id shield havingan aperture for passage of ions from the ion source to the target, and adetection means for detecting alpha particles emitted by the target ismounted within the said field-free zone in direct view of the target.

In this way Paschen breakdown between the alpha counter and the ionsource is prevented, and secondary electrons given off by the targetcanno-t attain full acceleration potential and impinge on the counter.

An embodiment of the invention is illustrated by way of example in theaccompanying drawing which is a diagrammatic general assembly.

In the drawing a sealed glass envelope 1 has an arm 2, containing adeuterium replenisher 3 as described in the first of the abovereferences, and an arm 4 containing a Pirani pressure gauge 5. A tritiumloaded target 6 is mounted within a metal cylindrical shield 7 by meansof brackets 8 and 9. A mesh 10 provides an aperture for ions directed atthe target. Cylinder 7 is supported on electrodes 11. A solid-statealpha counter 12 is mounted in a metal shield can 13 having an aluminiumfoil window 14, the window being opposite a port in cylinder 7 and beingin full view of the target 6.

The can reduces pick up from plasma oscillations in the ion source andfrom RR waves originating in power supplies. The aluminium foil window(0.9 mgm/cm?) allows entry of the alpha particles but stops light andions elastically scattered from the target.

A cathode 15 has a heater 16 both supported by their stilt wireconnections 17 and 18 respectively. A tubular anode 19 is supported onits connection 20. An extractor cathode 21 has a mesh 22 opposite themesh 10, the distance between the two meshes constituting anacceler-ation gap.

A coil not shown, applies an axial magnetic field to the space betweencathodes 1'5 and 21.

With the geometry shown, the nu-mber of alpha particles counted is aboutfi of the total.

To operate the tube, the target and alpha counter are A held .at earthpotential and a positive voltage of 60 kv. is applied to the extractorcathode, the anode voltage being about v. higher. With gas pressure of20 microns and an axial magnetic field of 200 gauss, the ion sourcecurrent was 0.5 to 1 ma, providing an ion current of 5 to 10 ma.molecular ions.

Alpha counter bias was 15 v. and amplified pulses can be fed to amulti-channel analyser and a timed scaler.

The alpha counter used in one embodiment was made by diffusingphosphorus into 3000 ohm p-type silicon. Electrical connection to thephosphorus difiused. surface was made by using a silver paste as a gluefor a fine wire.

We claim:

1. A neutron generator comprising a sealed envelope containing anelectrostatic shield positioned between a target and an ion source toform one boundary of an accelerating gap and to provide a field-freezone on the side of the shield remote from the ion source, said targetbeing located within said zone, said shield having an aperture forpassage of ions from the ion source to the target, and means mounted indirect view of the target for detecting alpha particles emitted by thetarget, said detecting means being mounted within said field-free zoneon the side of the shield remote from the ion source to preventsecondary electrons emitted from the target from being acceleratedtowards the detecting means.

2. A neutron generator as claimed in claim 1 wherein the detecting meansis a solid-state alpha-counter located behind a window which allows theentry of alpha particles to the counter while preventing the entry oflight thereto.

3. A neutron generator as claimed in claim 1 wherein the electrostaticshield is a tube, said aperture for passage of ions from the ion sourcebeing at one end of the tube and of smaller diameter than the tube, saidtarget being mounted within said tube at an angle to the direction ofthe ion beam and said detecting means being mounted in a side-arm sealedto the envelope, a port being provided in the wall of the tube to allowalpha particles from the target to reach the detector.

References Cited by the Examiner OTHER REFERENCES Hawkins et al.,Compact Pulsed Generator of Fast Neutrons, The Review of ScientificInstruments, vol. 31, No. 3, March 1960, pages 241-248. ScientificLibrary, No. Q 184 R5, copy in 313-61.

JAMES W. LAWRENCE, Primary Examiner. DAVID J. GALVIN, R. JUDD, AssistantExaminers.

1. A NEUTRON GENERATOR COMPRISING A SEALED ENVELOPE CONTAINING ANELECTROSTATIC SHIELD POSITIONED BETWEEN A TARGET AND AN ION SOURCE TOFORM ONE BOUNDARY OF AN ACCELERATING GAP AND TO PROVIDE A FIELD-FREEZONE ON THE SIDE OF THE SHIELD REMOTE FROM THE ION SOURCE, SAID TARGETBEING LOCATED WITHIN SAID ZONE, SAID SHIELD HAVING AN APERTURE FORPASSAGE OF IONS FROM THE ION SORUCE TO THE TARGET, AND MEANS MOUNTED INDIRECT VIEW OF THE TARGET FOR DETECTING ALPHA PARTICLES EMITTED BY THETARGET, SAID DE-